This invention relates to a method of laying a pipeline and to a pipelaying vessel. Such methods and vessels are of particular use in the offshore industry relating to oil and gas production.
One of the tasks that is carried out by certain vessels in the offshore industry is that of laying pipelines, either by S-laying or by J-laying.
In S-laying, the pipeline leaves the vessel at little or no inclination to the horizontal, adopts a steeper inclination in the water and then returns to a generally horizontal disposition on the seabed. The tension in the pipeline is often accommodated by a series of track tensioners mounted along the pipelaying path, on the vessel. The track tensioners support the weight of the pipeline and control passage of the pipeline. To allow S-laying a stinger may be provided, extending away from an end of the vessel with the pipeline passing over the stinger as it is laid from the vessel and increasing in its inclination to the horizontal as it passes along the stinger.
In J-laying, the pipeline leaves the vessel at a steep or vertical inclination and the inclination steadily reduces until the pipeline is in a generally horizontal disposition on the seabed. J-laying usually involves lowering a pipeline down a J-lay tower mounted on the vessel. The J-lay tower may be fixed and vertical or may be able to be inclined to the vertical. J-laying is used primarily for laying pipeline in deep water (that is typically deeper than 1,000 m).
A pipeline is laid on a seabed along a predetermined path with a certain tolerance allowed to either side of a centre line of the path. The area within the boundaries of the tolerance is commonly referred to as the “pipelaying corridor” or “laying corridor”. Typically a pipelaying vessel on the surface of the sea is aligned above the centre of the pipelaying path and is propelled along that path by the vessel's propulsion system as the pipeline is laid. In the case of S-laying, the pipeline typically leaves the vessel in the region of its stern aligned with the longitudinal axis of the vessel and at a downward inclination to the horizontal. In the case of J-laying the pipeline typically leaves the J lay tower in a vertical direction or at a steep inclination to the horizontal.
In some circumstances, for example certain wave conditions, it becomes desirable for the orientation of the vessel to be adjusted such that its longitudinal axis is inclined to the pipelaying corridor, for example to allow it to face into oncoming waves. When that occurs, it may be necessary to cease pipelaying, but that is undesirable. When J laying, especially in the case where the pipeline leaves the J laying tower in a vertical or almost vertical direction, it becomes possible to rotate the vessel about the J laying tower to a chosen orientation with little or no change to the path followed by the pipeline from the vessel to the point at which it touches down on the seabed. Modern vessel propulsion systems are versatile and it is straightforward still to propel the vessel along a path over the pipelaying corridor, even though the vessel is facing in a direction inclined to that path. A procedure of this kind, which is known as weathervaning is for example described in WO2007/094655 and can be adopted without undue difficulty.
In the case of S-laying, however, the position is very different. The generally shallower inclination of the pipeline as it leaves the stinger makes any such weathervaning a much more challenging prospect. It is in principle possible to introduce a curvature of the stinger structure about a vertical axis to introduce bending of the pipeline about a vertical axis. There is, however, then still a problem that, if the orientation of the vessel is rotated to face in a direction inclined to the pipelaying path, then the path followed by the pipeline from the downstream end of the stinger to the point at which the pipeline touches down on the seabed inevitably changes substantially and becomes complex. Also the stinger has to accommodate lateral forces exerted by the pipeline. In WO 2007/094655, this problem is recognised and the solution proposed for S-laying is shown in FIG. 18 and is to provide a stinger of sufficient length and curvature that the pipeline reaches its position of maximum (near vertical) inclination (inflection point) before it has left a flared guide assembly provided at the downstream end of the stinger. In this way the last portion of the stinger is actually acting in a manner similar to a J-lay tower and so the vessel is able to weathervane in the same manner as might a vessel with a J-lay tower, with the vessel rotating about an axis passing through the flared guide assembly (equivalent to the J-lay tower) on the stinger. In that case, as in conventional J-laying, the path followed by the pipeline from the downstream end of the stinger to the point at which the pipeline touches down on the seabed is substantially unaffected. This arrangement avoids the problem referred to above with conventional S-laying but can only be employed in relatively deep water
It is an object of the invention to provide an improved method of S-laying a pipeline and an improved pipelaying vessel for S-laying a pipeline.